The multi-cylinder internal combustion engines of motor vehicles often include at least one cylinder head connected to the mounting face of a cylinder block and four cylinders arranged in line along the longitudinal axis of the cylinder head, wherein each cylinder is equipped with ignition devices to initiate external ignition. Each cylinder generally contains at least one exhaust port to discharge the exhaust gasses from the cylinder via the exhaust gas discharge system, wherein each exhaust gas pipe is connected at each exhaust port. In the context of the following specification the term “engine,” in particular, comprises petrol engines equipped with external ignition. Engines have at least one cylinder head and one cylinder block which are connected together at their mounting faces to form the individual cylinders referred to as combustion chambers.
The cylinder head frequently serves to hold the valve actuating mechanism called the valve gear to control the charge change. In charge change, combustion gasses are expelled via the exhaust ports and a fresh mixture or fresh air is drawn in via the inlet ports, filling the combustion chamber. Reciprocating valves are often used as charge change control elements during operation of the engine to open and close the inlet and exhaust ports, wherein the aim is rapid opening of a flow cross-section large enough to keep choke losses low and maximize the fill of the cylinders. Therefore, cylinders are frequently fitted with two or more inlet or exhaust ports. Downstream of the manifold the exhaust gasses may then be sent to a turbine of the exhaust turbocharger and/or to one or more exhaust post-treatment systems. Power released by combustion is adjusted by changing the fill of the combustion chamber by adjusting the pressure in the aspirated air and varying the aspirated air mass. Lower loads rely upon a higher choking, so charge change losses are increased the low load region.
One approach for dechoking the working process of the petrol engine lies in the use of a variable valve gear with which the stroke of the valves and/or the control times can be varied to a greater or lesser extent. Varying the control times of the valves is achieved by use of a camshaft adjustment device with which the camshaft can be twisted through a certain angle in relation to the crankshaft allowing control times to be advanced or retarded without varying the opening duration of the valves. In this method of variable valve control, valve overlap depends on the crank angle range in which the exhaust is not yet closed while the inlet remains open. During valve overlap at high loads “flushing losses” can occur, wherein part of the aspirated fresh air flows through the cylinder without participating in the subsequent combustion. A variable valve control allows decreasing the valve overlap in response to increased rotation speed. For engines charged by means of exhaust turbocharging, at low rotation speeds a large valve overlap is suitable for raising the maximum torque and improving the unstable operating behavior. A pressure fall present at low rotation speeds between the inlet side and exhaust side supports an effective flushing of the cylinders with fresh air and ensures greater cylinder filling and hence higher power. A large valve overlap, possibly from late closure of the at least one exhaust valve, is also suitable for reducing the pumping and the resulting charge change losses.
Charge change has proved problematic for the exhaust pipes of the cylinders. Degradation can occur from the respective exhaust port through to the collection point in the exhaust gas discharge system at which the exhaust pipes merge into a common exhaust pipe and the hot exhaust gas from the cylinders is collected, this is compounded by the increasingly shorter exhaust pipe designs in modern engines. Increasingly often, the exhaust gas discharge system is integrated, at least partly, in the cylinder head in order to participate in the cooling provided in the cylinder head and reduce the need for expensive thermally heavy duty materials. Short exhaust pipes can lead to a mutual disadvantage of the cylinders of the engine on the effect on charge change, in particular, the effect achieved by residual gas flushing may be decreased. Thus in an in-line engine operated in a combustion sequence, the charge change of a cylinder can have a disadvantageous effect on the cylinder immediately preceding it in the ignition sequence due to different mechanisms competing to evacuate exhaust gas.
For example, exhaust gas emerging from the one cylinder entering another cylinder before its exhaust valves close resulting in two different mechanisms competing to evacuate combustion gasses from the fourth cylinder. Various approaches may be used to combat the problem arising from the short exhaust pipes, these approaches include shortening the opening duration of the exhaust valves by opening a valve later or closing a valve sooner. Use of large valve overlap is often heightened at low engine speeds by opening the valve later while maintaining closing time, this measure maintains engine torque at low engine rotation speeds; however, power disadvantages arise from shortened valve duration at high engine rotation speeds from the reduction in pumping in the low load range to reduce fuel consumption.
The inventors herein recognized this problem inherent in shortened exhaust pipes and recognized that some of the issues addressed above by providing some degree of isolation of the exhaust pipes of cylinders adjacent in the ignition sequence. Further, this method would alleviate the problems of mutual influence of adjacent cylinders on charge change while maintaining the benefits of a large valve overlap or long exhaust opening duration o minimize the power disadvantages arising with at high rotation speeds and/or with regard to the reduction of pumping in low load operation.